Solid state force transducer, support and method of making same

ABSTRACT

Solid state folded leaf spring force transducers are fabricated by batch photolithographic and etching techniques from a monocrystalline material, such as silicon. The folded leaf spring structure includes elongated gaps separating adjacent leaf spring leg portions, such elongated gaps being oriented parallel to a crystallographic axis of the monocrystalline material. In a preferred embodiment the monocrystalline material is of diamond cubic type and the leaf spring gaps extend in mutually orthogonal directions parallel to the &lt;011&gt; and &lt;011&gt; crystallographic axes, respectively. In a preferred method of fabricating the spring structure, the structure is etched from a monocrystalline wafer by means of an anisotropic etchant so as to more precisely define angles and dimensions of the resultant spring structure. In one embodiment, the gaps between adjacent leg portions of the spring structure are sealed in a fluid tight manner by means of oxide membranes left intact upon etching of the spring structure. In an accelerometer embodiment, sensing masses of equal weight are affixed to opposite sides of the spring structure for dynamically balancing same.

RELATED CASES

A force transducer employing a plurality of E-shaped folded cantileverleaf spring portions angularly displaced with respect to each otherabout an axis of sensitivity is disclosed and claimed in copending U.S.application Ser. No. 586,892 filed June 16, 1975, invented by BarryBlock and assigned to Diax Corporation. Furthermore, a solid state forcetransducer of the leaf spring type is disclosed and claimed by BarryBlock in application U.S. Ser. No. 656,632 filed on Feb. 9, 1976simultaneously with the present case. Sole inventors Block and Youmanshave assigned their interests respectively to assignees Diax andSignetics under development agreement.

BACKGROUND OF THE PRESENT INVENTION

The present invention relates in general to solid state forcetransducers and more particularly to an improved force transducer andmethod of making same wherein a folded leaf spring force transducerstructure is formed of a monocrystalline material.

DESCRIPTION OF THE PRIOR ART

Heretofore, monocrystalline force transducers have been fabricated inbatches by etching thin diaphragms out of a monocrystalline siliconwafer. Piezoresistive sensing elements were deposited on the surface ofthe diaphragm near the outer periphery thereof. One of thepiezoresistive elements was oriented so as to detect the radialcomponent of stress in the diaphragm and the second piezoresistiveelement was oriented to sense circumferential elements of stress in thethin membrane or diaphragm when the diaphragm was subjected to a force,such as that exerted by means of a fluid pressure differential acrossthe diaphragm. Such force transducers are disclosed in U.S. Pat. Nos.3,417,361 issued Dec. 17, 1968 and in 3,757,414 issued Sept. 11, 1973.In addition, this type of transducer is disclosed in an article titled"Solid State Digital Pressure Transducer" appearing in the IEEETransactions on Electron Devices, Vol. ED-16, No. 10, Oct. 1969, pages870-876.

One of the problems encountered in these prior art diaphragm typetransducers is that the strain sensors are located near the outerperiphery of the membrane where the maximum strain is the radial strainand where the tangential strain is near zero. However, location of thesensing elements near the outer periphery of the diaphragm, particularlyin the case where the periphery of the diaphragm is supported from arectangular support structure, introduces some undesired edge effectsresulting in a slight degree of nonlinearity of the force transducer.

It has also been proposed to construct an accelerometer by affixing amonocrystalline silicon beam to a support structure so as to achieve acantilever spring. A piezoresistive strain sensing element was depositedon the surface of the silicon beam for sensing the stress produced as aresult of a force exerted on the beam, such as by acceleration. Such anaccelerometer is disclosed in an article titled "A Silicon IntegratedCircuit Force Sensor" appearing in the IEEE Transactions on ElectronDevices, Vol. ED-16, No. 10, Oct. 1969, pages 867-870. See also IBMTechnical Disclosure Bulletin titled "Force Transducer" appearing inVol. 7, No. 12, of May 1966 at pages 1225-1226.

A problem with this cantilever structure is that the spring structurehas both rectilinear and a curvilinear displacement in response to theapplied force so that the detected strain is not a linear function ofthe applied force.

It is also known from the prior art to construct an accelerometer from aplurality of E-shaped spring structures so as to achieve rectilinearmotion of the sensing mass in response to a component of force appliedto the spring structure along the sensing axis. Such an accelerometer orforce transducer is disclosed in U.S. Pat. No. 2,702,186 issued Feb. 15,1955.

Furthermore, it is known from the prior art to provide a linear springsuspension structure for deriving rectilinear translation of a devicesuspended from the structure. The suspension system employs a pluralityof E-spring structures coupled to the displaceable device in a commonplane and in mutually opposed relation, i.e., disposed on opposite sidesof the displacement axis. Such a structure is disclosed in U.S. Pat. No.3,295,803 issued Jan. 3, 1967.

The problem with these prior art E-spring structures is that they arerelatively large, employing a number of elements which are joinedtogether via screws, or other fixing means with the result that they arerelatively costly of manufacture and in addition exhibit undesiredhysteresis effects.

It is also known from the above simultaneously filed Block applicationto fabricate a solid state leaf spring force transducer from a siliconwafer by photolithographic techniques previously developed for thesemiconductor industry.

SUMMARY OF THE PRESENT INVENTION

The principal object of the present invention is the provision of animproved solid state leaf spring force transducer and more particularlyto such a transducer which is advantageously fabricated employingphotolithographic techniques as developed in the semiconductor industry,whereby the manufacturing cost and size are substantially reduced and insome instances the performance substantially improved over the priorart.

In one feature of the present invention, a folded leaf spring forcetransducer structure is etched from a wafer in such a way as to form arecess leaving an unetched base support structure at least partiallysurrounding and connected to the leaf spring structure in supportiveengagement therewith.

Other features and advantages of the present invention will becomeapparent upon a perusal of the following specification taken inconnection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinally foreshortened transverse sectional view of awafer of monocrystalline material employed in the process of the presentinvention,

FIG. 2 is a view similar to that of FIG. 1 depicting an oxide layerformed on opposite sides of the wafer,

FIG. 3 is a plan view of the wafer of FIG. 2 showing a pattern of foldedleaf spring structures formed in a photoresist coating over the surfaceof the wafer,

FIG. 4 is an enlarged detail view of a leaf spring pattern delineated byline 4--4 of FIG. 3,

FIG. 5 is an enlarged sectional view of a portion of the structure ofFIG. 4 taken along line 5--5 in the direction of the arrows,

FIG. 6 is a view similar to that of FIG. 5 showing a subsequent step inthe process for fabrication of the spring structure of the presentinvention;

FIG. 7 is a view similar to that of FIG. 6 showing a subsequent step inthe process,

FIG. 8 is an enlarged sectional view of a portion of the structure ofFIG. 7 delineated by line 8--8 and depicting a subsequent step in theprocess wherein piezoresistors are formed in the surface of the springstructure,

FIG. 9 is an enlarged detail view of a piezoresistor formed in thesurface of the spring structure,

FIG. 10 is a view similar to that of FIG. 7 depicting a subsequent stepin the process,

FIG. 11 is an enlarged sectional view of a portion of the structure ofFIG. 10 delineated by line 11--11 and depicting a subsequent step in theprocess,

FIG. 12 is an enlarged view similar to that of FIG. 10 depicting etchingof a recess in the wafer from the back side to define the springstructures therein,

FIG. 13 is a bottom view of the etched structure of FIG. 12 taken alongline 13--13,

FIG. 14 is an enlarged sectional view of a portion of the structure ofFIG. 13 taken along line 14--14,

FIG. 15 is a top view of the structure of FIG. 12 taken along line15--15 in the direction of the arrows,

FIG. 16 is a schematic sectional view of a dual in-line integratedcircuit package having the force transducer of the present inventionmounted therein,

FIG. 17 is a plan view similar to that of FIG.. 15 showing analternative spring structure of the present invention, and

FIG. 18 is an enlarged sectional view of a portion of the structure ofFIG. 12 delineated by line 18--18 and depicting a compliant sealingmembrane formed between adjacent leg portions of the spring structure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1 there is shown a typical wafer 11 from which abatch of force transducers are to be fabricated according to the processof the present invention. In a typical example, the wafer 11 is made ofa nonmetallic monocrystalline material such as silicon, germanium,quartz, gallium arsenide, gallium phosphide, etc. In a preferredembodiment, the wafer 11 is made of a diamond cubic material, such assilicon and the wafer 11 has a thickness of 10 mils or 254 ± 2 micronsand a convenient diameter, such as 3 to 5 inches. In the case of diamondcubic material, the crystallographic plane is preferably formed at theupper and lower major surfaces of the wafer 11. Furthermore, the wafer11, in the case of silicon, is preferably doped with an N type dopantsuch as phosphorus to a resistivity of 6 to 8 ohm centimeters. In thesecond step of the process, the wafer 11 is oxidized on opposite sidesto form oxidized layers 12 and 13, as of 8000 angstroms in thickness.This is conveniently achieved by putting the wafers in a furnace at1150° C. in the presence of oxygen.

In the next step of the process, the oxide layers 12 and 13 are coatedon the top side with a photoresist material and on the bottom side witha protective coating, as of krylon. The photoresist coating is exposedby a conventional photolithographic process to an array of transducerpatterns, such as the double E-shaped folded cantilever spring pattern15 of FIG. 4. Furthermore, each of the individual patterns 15 isseparated from the adjacent ones by means of rectangular boundarypattern portion or frame 16. After exposure of the photoresist coating14, it is developed and then etched out to expose the oxide layer 12.The oxide layer is then etched through the slot pattern 15 formed by thephotoresist coating by means of a buffered hydrofluoric acid etchant todefine the spring patterns 15 and frame patterns 16 in the oxidecoating, thereby exposing the silicon wafer through those patterns.

Next, the silicon wafer 11 is etched with an anisotropic etchant such as25% by weight of sodium hydroxide in water. The spring and framepatterns 15 and 16 were aligned to the <110> crystallographic plane ofthe wafer, as shown in FIG. 3, with the lines of the patterns which areto form spring defining gaps or cuts in the wafer structure, orientedparallel to either the <011> crystallographic axis or the <011>crystallographic axis. The proper orientation relative to the wafer 11is determined by a chord 19 sliced from each wafer, such chord 19running perpendicular to either the <011> or the <011> crystallographicaxis.

The anisotropic etchant etches preferentially along the <111>, <111>,<111> and <111> planes, thus precisely defining the gaps or grooves inthe surface of the wafer 11 which are to subsequently define the springstructure of the force transducer. In a typical example of anaccelerometer, the anisotropic etch is continued to a depth ofapproximately 25 microns in the upper surface of the monocrystallinewafer 11. In the anisotropic etching step, the krylon protective coating17 is also etched from the oxide layer 13 on the bottom of the wafer. Inthe next step, as shown in FIG. 6, the krylon coating 17 isreestablished on the bottom and then the oxide layer 12 is stripped fromthe upper surface of the wafer 11 by means of etching in dilutehydrofluoric acid. Next, the krylon coating 17 is stripped from thebottom surface by means of a conventional stripper such as phenol.

In the next step as shown in FIG. 7, the upper surface of the wafer 11is reoxidized to form layer 21 to a thickness of approximately 1600angstroms. The oxide layer 21 is then coated with photoresist materialand exposed via conventional photolithographic exposure techniques to apattern of radiation corresponding to a plurality of piezoresistors tobe formed in the surface of the resultant spring structure or pattern15. Typically, the piezoresistors are generally of the pattern shown inFIG. 9.

One set of the piezoresistors are oriented with the longitudinal axes ofthe piezoresistors extending longitudinally of the spring leg portionsof the spring structure in a region thereof of maximum stress, such asnear the point of support of one of the inner ends of the outer legportions. A second set of piezoresistors is also formed in that regionwith their longitudinal axes aligned perpendicular to the longitudinalaxis of the particular leg of the spring so as not to pick up a changein piezoresistance due to stress of the spring structure caused bydisplacement thereof in response to a force applied to the spring alongits axis of sensitivity, i.e., into the paper in FIG. 4.

These two orientations of piezoresistors are utilized in an electricalbridge circuit so that the difference between the piezoresistance yieldsa measure of the stress in the spring structure and thus a measure ofits displacement in response to a force applied thereto to be measured.

Next, the photoresist layer 22 is developed to expose the oxide layer 21in accordance with the pattern of the piezoresistors to be formed. Next,the back surface of the wafer is coated with krylon, the photoresist isremoved in the exposed piezoresistor pattern to expose the underlyingoxide layer 21 in accordance with the developed patterns 23. Then theoxide layer 21 is etched through with dilute hydrofluoric acid to exposethe underlying monocrystalline silicon wafer 11. Then the wafer isstripped of the krylon and the remaining photoresist.

Next, as shown in FIG. 8, boron is diffused into the silicon wafer 11through the openings 23 in the oxide layer 21. The boron diffusion isobtained by contacting the surface of the silicon wafer 11 with a boronnitride wafer causing the boron nitride wafer to dissociate and thefreed boron to diffuse into the wafer to form the piezoresistor, suchpiezoresistors having a resistivity of 300 ohms per square. Next, thepiezoresistor layer 25 is covered over by regrowing a thin layer ofoxide 24 over the piezoresistors, such oxide being obtained by exposingthe wafer 11 at elevated temperature to a carrier gas of oxygen andnitrogen.

Next, the wafer 11 is coated on the top surface with a photoresistmaterial and exposed to a pattern of radiation corresponding to contactapertures to be made to the opposite ends of each of the piezoresistors25, as shown at 26 in FIG. 9. The photoresist material is then developedand removed in the regions through which contact is to be made. Next,the thin layer 24 of oxide is etched by dilute hydrofluoric acid toexpose the piezoresistors 25 at the contact regions 26. During the etchstep, the back surface of the wafer is protected by a coating of krylon.Next, the photoresist material is stripped and aluminum is evaporated toa thickness of approximately one micron over the front surface of theoxide coated wafer, such aluminum making contact to the piezoresistors25 through the contact openings 26. Then, a photoresist coating isapplied over the aluminum and exposed to a pattern of radiationcorresponding to the intraconnect lead pattern to be formed on thewafer. The photoresist material is then developed to expose the aluminumin the desired regions and then the aluminum is etched with phosphoricacid. Next the wafer is placed into an oven at 500° C. to alloy thealuminum contacts into the piezoresistor regions 25.

Referring now to FIG. 10, next, the bottom side of the wafer 11 iscoated with a photoresist material 31 and exposed to a pattern ofradiation corresponding to a base support structure which is to surroundeach of the spring structures. The base support structure is shown inFIG. 13. The photoresist material 31 is then developed and removed inthe regions which are to be etched. The top side of the wafer 11 is thencoated with krylon and the oxide layer 13 on the bottom side is etchedthrough by means of dilute hydrofluoric acid to define the basestructure.

Next, as shown in FIG. 11, the bottom surface of the wafer 11 is coatedwith a first layer of chromium 32 to a thickness of approximately 500angstroms followed by a layer of gold 33 to approximately 8000 angstromsin thickness. Next, the metallized layers are coated with a photoresist,exposed to patterns of radiation corresponding to the base structure.The front side of the wafer 11 is protected by krylon and then the backside is etched with an etchant for the gold and chromium layers.

Next the wafer, as shown in FIG. 12, is exposed to an anisotropicetchant for the silicon, such etching producing a recess in the backside of the wafer which extends inwardly to an intersection with thespring defining pattern of recesses in the front surface of the wafer,to define the spring structure and its base support as shown in FIG. 13.More particularly, the resultant spring structure includes a pair ofmutually opposed E-shaped springs 35 and 36 each having a pair of outerleg portions 37 supported at their inner ends from inwardly directedportions of the base support 38. The outer legs 37 are interconnected attheir outer ends to the outer end of a central leaf spring portion 39.

In a preferred embodiment, two transverse strengthening members 41 areformed at the outer ends of each of the E-shaped spring portions 35 and36 by stopping the etching at an intermediate point in the formation ofthe back recess, coating the strips 41 and 42 with a suitable protectivematerial such as cr and gold and then continuing the etch.

A sensing mass 42 is affixed to both the top and bottom surfaces of thecentral leg portions 39 of the E-shaped leaf spring 35 and 36, as shownin FIG. 14 and in FIG. 13, to facilitate rendering the composite springstructure responsive to acceleration. In a typical example, the masses42 comprise two 10 milligram squares of gold affixed as by glue to theinner ends of the central leg portions 39 of the E-springs 35 and 36.

When the etching is complete the individual accelerometer or forcetransducers are individually etched out of the wafer. The wax is thenremoved from the front surface of the transducers and they are mountedin a conventional dual in-line integrated circuit package as shown inFIG. 16.

More particularly, a base plate 44 is affixed to the upper surface ofthe inner ends of the lead frames 45. The transducer 46 is die attachedto the upper surface of the mounting plate 44 as by conventional dieattach techniques utilizing the gold and chromium coating on the bottomsurface of the base support structure of the individual force transducer46. Wire bonding leads 47 are then wire bonded between the individualconductors of the lead frame structure 45 and the respective connectingpads on the front surface of the transducer 46. The front surface of thetransducer 46 is shown in greater detail in FIG. 15.

In a typical example, the resultant transducer 46 has a spring thicknessof approximately 25 microns, a length of approximately 200 mils and awidth of 150 mils and is approximately 10 mils thick in the region ofthe surrounding base support structure.

Referring now to FIG. 17, there is shown an alternative spring patternconsisting of four E-springs 35 located at 90° angles about the centralaxis of sensitivity 40, which is perpendicular to the paper. The springstructure of FIG. 17 is very rigid in response to forces applied normalto the axis of sensitivity. Thus, there is relatively little crosscoupling of forces out of the axis of sensitivity into deflection of thestructure along the axis of sensitivity.

Referring now to FIG. 18 there is shown a structure for sealing thespring structure in a fluid tight manner while permitting deflection ofthe spring structure. More particularly, the silicon dioxide coating 21,which was formed on the top surface of the wafer during the stepdepicted in FIG. 7, is protected, in the region of the slots, throughthe various processing steps. In the last etching step, as depicted inFIG. 12 the anisotropic etchant does not attack the silicon dioxide,thereby leaving a thin silicon dioxide web interconnecting adjacent leafspring portions and bridging the gap therebetween. The silicon dioxideweb would ordinarily be permeable to water vapor and thus is coated witha suitable sealant such as a layer of tantalum or gold, or parylene.

The advantages of the present invention include the batch fabrication oftransducers of the folded spring type which exhibit improved linearityas contrasted with the prior art diaphragm and cantilever forcetransducers. An accelerometer of the configuration of FIGS. 13 and 15has yielded an output signal of 20 volts peak-to-peak when tiltedthrough an angle of 360° relative to the earth's gravitational field.The measured peak-to-peak deviation of the output signal from G sin θ,where θ is the angle of tile relative to the gravitational horizontaland G is the earth's gravitational force, was only 0.1 percent of thepeak-to-peak full scale output.

As used herein, monocrystalline is defined to include an epitaxial layergrown upon a monocrystalline substrate even though the expitaxial layermay not comprise only a single crystal.

I claim:
 1. In an integral transducer element and support structure, asemiconductor body having top and bottom surfaces, said body having arecess formed therein extending upward within said body from said bottomsurface to define internal side walls and a top wall, the top wall ofsaid recess being spaced from said top surface to form a relatively thintransducer membrane portion within said body at least partiallysurrounded and supported by said internal side wall portions of saidbody, said transducer membrane having spaced elongate slots formedtherethrough extending from said top wall of said recess to said topsurface and being positioned intermediate the side walls of said recessextending to said top surface and terminating short of said side walls,said body having a slot formed about the periphery of said membraneportion at the junction of the recess side and top walls extending tosaid top surface with portions of said body remaining to integrallysupport the transducer element thereby formed.
 2. A structure as inclaim 1 wherein said semiconductor body top surface is a substantiallyplanar surface.
 3. A structure as in claim 1 wherein said portions ofsaid body remaining for support are confronting portions approximatelymidway along the elongate axis of said spaced slots.
 4. In a method forfabricating a semiconductor force transducer, providing a semiconductorbody having top and bottom surfaces, forming a recess having side wallsand a top wall in said bottom surface extending upward within said bodyand spaced from said top surface to define a transducer membrane portionwithin said body at least partially surrounded and supported by saidside wall portions of said body, forming spaced elongate slots extendingthrough said membrane portion and forming a slot extending around theperiphery of said membrane with portions of said body remaining tointegrally support the transducer element thereby formed.